Australian captain Michael Clarke revealed that the scars of the painful loss against South Africa in the first Test have healed.

"I think we got rid of them (the mental scars), to be honest, after we left the change room after the day's play in (Cape Town)," Clarke said. "We had a really long chat after the game and we made a pact with each other that once we left there, the ground, that we were going to look to this second Test."

It's a simple equation for Australia, who must win the second Test to level the contest and save their long-standing unbeaten run here. They have won their last four series in South Africa and last lost here in the apartheid era.

"As a captain and as a player ... you want to win every game that you play. Well, we have no choice out there. We have to pick a team to try and take 20 wickets," Clarke said.

"We also have to pick a team that is going to be positive and confident with the bat as well, and be willing to take the game forward. We have no choice."

Australia have already been forced to call in the inexperienced Usman Khawaja to bat at No. 3 in place of Shaun Marsh, who went home with back problems.

"There is the possibility of a couple of other changes as well," Clarke said, suggesting players could be dropped and pay for that lackluster defeat at Newlands.

Teenager Pat Cummins is raring to be given a debut, putting Johnson, Ryan Harris and Peter Siddle under pressure after Clarke backed the 18-year-old on the eve of the match.

"He's (Cummins) done everything right, he's keen as mustard for an opportunity," Clarke said. "He's bowled well in the nets. For some unknown reason, I seem to face him every time I'm in the nets as well, so I think he's keen to bowl at the skipper and let him know he's ready."

"Yeah, I don't want to give too much away right now but there's certainly a chance that he could be playing tomorrow (Thursday)."